Growth and reworking of continental crust: Insights from the Dunhuang Block, NW China

Abstract

<p>The craton margins and small-scale Precambrian blocks are not always with stable lithosphere; they tend to lose their roots and be reworked and isotopically reset by subduction and collision from multiple directions. The Dunhuang Block is one of the microcontinents within the Central Asian Orogenic Belt (CAOB), and has been involved and reworked by tectonics evolution of the southern CAOB, leading to extensive reworking and recycling of continental lithosphere. Therefore, the Dunhuang Block is an excellent example for revealing the growth and reworking of early Precambrian continental crust. Archean–Paleoproterozoic basement rocks in the Dunhuang Block are sporadically exposed and spatially associated with Paleozoic complexes related to the CAOB. Available geochronological data reveal that the Dunhuang Block is a unified Precambrian block that likely formed a coherent crystalline basement prior to ca. 1.79 Ga. In the representative Gangou cross-section of the Dunhuang Block, the early Precambrian rocks were mainly composed of Neoarchean to Paleoproterozoic TTG gneisses with a small amount of late Paleoproterozoic pyroxenite and marble. The protoliths of Neoarchean TTG rocks in the Dunhuang Block were likely originated from a subducted oceanic slab under the garnet-amphibolite- and rutile-eclogite-facies conditions with presence of rutile; whereas the Paleoproterozoic TTG rocks likely originated from partial melting of thickened mafic lower continental crust under amphibolite-facies condition. Hf isotopic compositions in zircons from the Precambrian rocks in the Dunhuang Block reveal that remnants of Hadean-Eoarchean crustal components were still present in the basement of the Dunhuang Block. The Neoarchean rocks in the Dunhuang Block have been mainly extracted from the depleted mantle, whereas the Paleoproterozoic rocks have been primarily reworked from older crustal reservoirs. The Dunhuang Block was an independent Precambrian continental fragment or microcontinent situated between the Tarim Craton and North China Craton. Similar to many other Precambrian blocks within the CAOB, it was significantly involved in Paleozoic orogenic events related to the subduction-accretion processes of the Paleo-Asian Ocean in the southern CAOB.</p>